I'm in need of an idea to advance my fictional crime plot where the protagonists of a larger (~30) group realize that the murderer (of a single murder) must be one of them, someone hiding in the group, playing innocent with everyone else. I am looking mostly for inspiration, short stories, novels that are built along similar lines, so that I can extract and bend the story elements according to my needs. To make it clear: I'm not only looking for direct ideas, but also for similar sources. So you might find it more of an identify-this-plot-type question than a purely opinion-based one.

The closest motif that is very similar is Asimov's short story "Little lost robot" from "I, robot" (not the movie). In this sci-fi novel, a robot gets the offhand instruction to "get lost", and he takes it seriously: he tries to merge within a larger population of 62 similar robots lying to any direct questioning about it's origin and intentions (thereby effectively defying the 2nd Law of Robotics: obeying humans --> a free-roaming, lying robot poses a serious threat). The protagonist robot psychologist (Susan Calvin), after hours of futile questioning, realizes that out of the 63 robots (62 normal, 1 lying), only one is prepared with a specific piece of knowlege (the one lying), but he himself does not know that the other 62 is lacking this knowlege. Calvin cleverly uses this discrepancy to force the lier to reveal himself by externalizing the internal difference to appear as a clearly visible difference in behaviour.

So abstracting away: I need ideas where the protagonists realize that

  • there is a nontrivial differentiating piece of information available for them (and for the Reader), which ensures that the otherwise undifferentiable murderer differs from the innocents, AND
  • this information must be hidden i.e. cannot be trivially available to reveal the murderer, AND
  • the murderer does not know this difference exists (thus the trivial solution "Are you the murderer?" wouldn't do any good), AND
  • the murderer can be trapped by exploiting the differentiating information in a way that if the murderer would react to it, no matter hows casually he would do it, would reveal the difference at an obvious level for the audience and for the Reader.

That is, during the confrontation, the murderer still reacts to the trap question according to his best intention to hide himself, assuming that he HAS to know this knowledge (as he believes all the other members of the group is in possession of this knowledge). Since the protagonists know, that no other member of the larger group should know about this information, the murderer is revealed, against his intention.

  • I think this falls afoul of our "asking what to write" rule. There might be a good question here, but I think this needs a bit of reworking for this to be on-topic. I'm closing for the moment, for discussion and editing. If we can improve, I'd love to reopen. – Standback Jan 12 '15 at 12:08
  • So, IMHO, the way to turn this into a workable question is to explain what precisely your problem is. You know what you're looking for. So, what's holding you back from coming up with something appropriate and running with that? If it's "I don't have any good ideas," then this remains off-topic - Q&A doesn't work for brainstorming story ideas. But if you do have ideas, but all of them are giving you trouble - then that's a problem we might be able to help with. – Standback Jan 12 '15 at 12:12
  • @Standback. I assumed that this will be closed, against all my cooperative intentions. Isn't it enough that I specifically stated that I look for sources (rather than ideas/opinions) that deal with similar problems? I don't expect other people to brilliantly solve what I cannot on my own. – István Zachar Jan 12 '15 at 13:16
  • I assure you, your effort in writing a clear, comprehensive question are evident and appreciated :) I'd be happy to discuss this further in chat. Also, community members with opinions are very welcome to weigh in, as always :) – Standback Jan 12 '15 at 13:37
  • @Standback I let this go (and answered it) because I thought it was broad enough to be applicable to others. It is on the border of discussion/what to write, but if I were to Google this and this question came up, I would find it useful. If anything, making it specific to the OP's story would make it too localized: "Help me fix my story," not "Help me develop a tool." – Lauren-Clear-Monica-Ipsum Jan 12 '15 at 14:29

Exposure to a chemical (or radiation, or some other toxin).

All the suspects are in a particular area, or do a particular task, but only the murderer gets exposed to the MacGuffinium. The suspects are screened in some manner, and the murderer submits to the screen, thinking all evidence has been cleaned away (using normal methods), but the screen which the protagonist uses is a level or two up, and picks up the contamination.

Not a precise example of this, but close, in House, MD, in the episode "Clueless": The wife was poisoning her husband but didn't know the poison would leave a chemical trace on her hands, and House was able to use something to demonstrate the poison on her hands.

On the Agent Carter episode "Bridges and Tunnels:"

Carter is exposed to radiation from an explosion, and realizes that her watch absorbed more of the radiation than her clothes and skin. When one of the bad guys comes up in the line but doesn't ping, she suggests that his locker, including his street clothes and watch, should be scanned. Bad Guy bolts.

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